Good thinking... hmmm
requires some post DriverPacks BASE editing... not normaly recomended, but is what you need if you are committed to going that way 
replace PRESETUP.CMD with one similar to this untested example...
Rem 'Dest' is your desired alternate extraction drive... D: has been set for you as an example
Set Dest=D:
If exist "%systemroot%\system32\setupold.exe" DEL /F "%systemroot%\system32\setupold.exe"
REM +==========================================================================+
REM | Finding CD/DVD driveletter. |
REM |--------------------------------------------------------------------------|
SET TAGFILE=\OEM
FOR %%i IN (C D E F G H I J K L M N O P Q R S T U V W X Y) DO IF EXIST "%%i:%TAGFILE%" SET CDDRIVE=%%i:& GOTO DPsFound
:DPsFound
REM +==========================================================================+
REM | Decompressing the DriverPacks to the harddisk - using un7zip.exe. |
REM |--------------------------------------------------------------------------|
Start /wait /realtime /separate "" %CDDRIVE%\OEM\bin\un7zip.exe %CDDRIVE%\OEM\DP*.7z %Dest%\
REM +==========================================================================+
REM | Copying/decompressing the files to finish the installation. |
REM |--------------------------------------------------------------------------|
%CDDRIVE%\OEM\bin\un7zip.exe %CDDRIVE%\OEM\bin\*.7z %SystemDrive%\
COPY /Y %CDDRIVE%\OEM\bin\DPsFnshr.ini %SystemDrive%\
IF EXIST %CDDRIVE%\OEM\*.ins COPY /Y %CDDRIVE%\OEM\*.ins %SystemDrive%\
REM +==========================================================================+
REM | Scanning for driverdirectories. |
REM |--------------------------------------------------------------------------|
%CDDRIVE%\OEM\bin\DevPath.exe %Dest%\D
REM +==========================================================================+
REM | Disable Driver Signing Policy and keep it disabled. |
REM |--------------------------------------------------------------------------|
START %SystemDrive%\DSPdsblr.exe
EXIT
it will also require an edit of the \OEM\bin\DPsFnshr.ini
[Settings]
; DO NOT SET THIS MANUALLY, LET THE DriverPacks BASE DO IT FOR YOU! (may NOT contain any spaced, if it does, all exceptions must be adapted to be compatible!)
REM DPsRoot = "%SystemDrive%"
DPsRoot = "D:"
; DO NOT SET THIS MANUALLY, LET THE DriverPacks BASE DO IT FOR YOU!
KTD = "false"
; system variables supported (see remarks for a list) one location cannot be used as
; KTDlocation: the root directory specified in[Settings]\DPsRoot
KTDlocation = "%SystemRoot%\DriverPacks"
; system variables supported (see remarks for a list)
logLocation = "%SystemRoot%"
; enables/disables debug mode (enabled by default)
debug = "true"
I think the finisher will handle it fairly well, however, certain exceptions will fail. (any that are hard-coded for 'C:\D\...')
Again this is not tested...
I think you could do fairly well this way - but is not the way that I would go.... 
---- alternate and probably a better solution ----
since you want a Disc that is specific to your machine, and only has to support your single unit...
I would just grab the handful of drivers you need and add them with nLite.
That is the quickest and easiest way to solve your space problems - and speeds up your install by ten minutes or more!
I like the way you think though, well done!
